This invention relates to a production method of machine parts such as a cylinder and a feeding screw of an injection molding machine which is used for injection-molding or extrusion-molding a plastic or a ceramic material and also to the machine parts thus produced.
Recently, injection molding machines and extrusion molding machines which have been used for molding plastic materials have been further utilized for molding ceramic materials and the like, and the application field of the machines is thereby much expanded. However, a part of the machine such as a cylinder or a feeding screw which is generally made of cast iron and constantly and directly in contact with the aforementioned material is easily corroded and worn out by the material.
Various methods have been proposed for eliminating the difficulties and one of the most ordinary method is characterized by the formation of a nitride layer on the surface of the machine part. However, the nitride layer thus formed is excessively thin and easily worn out by the material to be used. According to another method, a member which is made of a corrosion-resistant material such as chromium cast iron, high-speed steel, tool steel, Hastelloy or the like is shrink-fitted, silver-brazed, or bonded by use of a bonding agent to an area of the machine part which is brought into contact with the plastic or ceramic material. In these cases, however, since the molding operation of the plastic or ceramic material is carried out at a temperature of 250.degree. C. to 350.degree. C. and heating and cooling operations are repeated, the shrinking stress of the shrinkage fit is adversely reduced. Moreover, in the case where the corrosionresistant member is bonded as described above, a bonded layer formed between the machine part and the corrosionresistant member tends to be cracked. Reduction of the shrinking stress or creation of cracks in the bonded layer reduces the heat transmission to the corrosion-resistant member, and hence the quality of the products of the injection molding machine and the like is made unsatisfactory. Furthermore, the wear and corrosion resistant member is in itself brittle, and therefore, the machine part shrink-fitted or bonded with this member becomes also brittle.
For obviating these difficulties, there is proposed a still further procedure wherein a self-fusing material or nickel or chromium series alloy is deposited in accordance with a metal spraying method or a centrifugal casting method for lining the portion of the machine part, that is, brought into direct contact with the plastic or ceramic material. Such a procedure, however, is hardly applicable to the machine part of a small diameter and a large length, and the lining technique based on the centrifugal casting method cannot be applied to a machine part other than a cylindrical configuration.